Security alarm system

ABSTRACT

A SYSTEM FOR PROVIDING A CENTRAL STATION WITH AN INDICATION OF UNAUTHORIZED ACTIVITY AT A REMOTE PROTECTED ARE WHICH INCLUDES A PULSE GENERATOR AT THE REMOTE AREA. THE PULSE GENERATOR TRANSMITS TO THE CENTRAL STATION DURING NONALARM CONDITIONS PULSES HAVING PRE-DETERMINED PEAK AND BASE AMPLITUDES FALLING WITHIN AN ACCEPTANCE BAND AND HAVING A SPECIFIED PULSE WIDTH TO PULSE SPACING RATIO. SHOULD AN INTRUSION OCCUR THE PULSE WIDTH TO SPACING RATIO OF THE GENERATOR OUTPUT CHANGES. A DETECTOR AT THE CENTRAL STATION INTERCONNECTING THE PULSE GENERATOR AND AN ALARM INDICATOR MONITORS THE PULSE PEAK AND BASE AMPLITUDES TO ASCERTAIN WHETHER THE PULSES FALL WITHIN THE ACCEPTANCE BAND AND TO THEREBY DETECT TAMPERING, AS WELL AS MONITORS THE AVERAGE DIRECT CURRENT LEVEL OF THE PULSES TO ASCERTAIN THE EXISTENCE OF AN INTRUSION.

United States Patent US. Cl. 335-192 6 Claims ABSTRACT OF THE DISCLOSURE A chattering-less electric relay, of which movable contacts are switched over in accordance with downward attraction and upward deattraction of an armature to a yoke through a card normally biased upwardly. The armature includes an adjustable biasing means to urge the card downwardly when the armature is being attracted and to urge the armature itself upwardly at both last stage of the attraction and initial stage of the deattraction. There is provided a small gap between the card and the biasing means during deattraction of the armature.

This invention relates to improvements in relays.

Conventional relays have in general the below enumerated defects in their structures.

First of all, particularly in a relay having an electromagnetic structure of a type in which the attraction becomes strong particularly at the final end point of the attracting operation of the armature, a nonmagnetic material has been inserted between the armature and yoke to prevent the returning characteristic from being deteriorated by such excess attraction. Therefore, the ampere turn of the electromagnet for operating the same load has been larger than in the case of using no nonmagnetic material and this has been a loss. In the present invention, by providing an adjusting spring which can additionally apply to the armature a returning force which becomes available just before the moment when the armature completely engage the yoke, the opening value between the armature and yoke is made to be freely adjusted and, yet, the attraction of the yoke is made to be secured at its maximum value, so that the above discussed problem will be effectively solved.

Second, the conventional relays have also in general a defect that, as the part for regulating the position of an element referred to as car at the time when the card is not excited and the part for mounting fixed contacts have been arranged so as to be separated from each other, any errors in the allowance for movement of the card and in contact gap could easily become so large due to accumulated errors between the card length, yoke thickness, fixed contact position and armature hinge position that a complicated adjustment has had to be made by applying a plastic deformation to each element.

According to the present invention, as the regulating surface of the above mentioned card and the fixed contact are combined in the same block, such errors of the card allowance and contact gap can be limited to be minimum. Third, in the conventional relays, as the armature and card have been directly connected with each other, any possible vibration of the armature of a large mass when it returns to its position in non-excitation state could be transmitted directly to the card and, thus, the chattering "ice between the contacts by the vibration of the card has been unavoidable.

According to the present invention, a clearance or a play is provided between the position in which the armature pushes the card and the position in which the armature is prevented by the card from moving upward, so that no such direct influence of the vibration of armature on the contacts will be caused to occur.

Further, as a spring bent substantially in the form of Z has been adopted in the conventional relays for hinging spring for the armature, the downward biasing force applied to the hinge part of the armature on the yoke has been substantially constant and weak during the attracting operation of the armature, so that not only the magnetic resistance at the hinged point has been made high, but also errors in angle or position of armature could have been easily involved due to required bending works for the spring.

According to the present invention, a fiat type spring is adopted for the hinge spring and, thus, the force applied to the hinged part of the armature can be increased as the attracting action by the yoke progresses, no bending Work of the spring is required, and such position errors of the armature can be entirely eliminated.

Yet further, as stopper means for upward rotation of the armature in conventional relays have been provided in a part separate from the card, component parts have been inherently increased in number.

On the other hand, in the present invention, the stopper means is formed integrally as a part of the card, so that the number of component part is decreased without affecting any of effects or functions of those components.

The present invention has been suggested to eliminate the above mentioned defects of the conventional relays.

A main object of the present invention is therefore, to provide a relay which is remarkably easy to be adjusted.

Another object of the present invention is to provide a relay which prevents effectively any chattering of movable contacts from occurring.

A further object of the present invention is to provide a relay in which a contact gap and card allowance are regulated by controllably regulating the card position and, therefore, it is not required to give any plastic deformation to a terminal plate Another object of the present invention is to provide a relay whose return characteristics are improved.

FIG. 1 is a partly sectioned perspective view of a relay of the present invention with a cover and base plate as removed.

FIG. 2 is a perspective view of the relay as disassembled of the present invention as shown in FIG. 1.

FIG. 3 shows a frame for coil and fixed contacts in the relay of FIGS. 1 and 2, FIG. 3A being a plan view and FIG. 3B being a front elevation.

FIG. 4A is a side view of the relay as assembled of the present invention and FIG. 4B is a sectioned view taken along line IVIV in FIG. 4A.

FIG. 5 is an explanatory view showing a method of assembling the respective components in FIG. 2.

FIGS. 6A through 6H show a sequence of an operational travel of the armature and sequential opening and closing states of contacts according to the present invention.

FIG. 7 shows diagrammatically the relationship of the operational travel of the armature to the attraction force of yoke and the resistance force of the armature.

Referring first to FIGS. 1 to 5, 1 is an E type yoke of a magnetic material provided with a central leg 2, side legs 3 and 3' and slots 4 and 4' outside said side legs 3 and 3, respectively. 5 is a gap between said central leg 2 and respective side legs 3 and 3'.

Jan. 5, 19711 M. KAPLIN ET AL SECURITY ALARM SYSTEM Filed Sept. 21. 1966 3 Sheets-Sheet 2 Qnw mmNNDm WWW J1 ATTORNEYS womnOm 0 d ma i-F200 womnow 0 d MDQDZFZOO l I I I l mom-Dom 0 d Pll Jan. 5, 1971 A M. KAPLIN ETAL 3,553,730

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7 TIME ZL: 5-8 772 'gE Tg I 3y MM A NONINTRUSION T' flfimem PULSES\ i-ToRNEys United States Patent 3,553,730 SECURITY ALARM SYSTEM Martin Kaplan, Westport, Donald E. Hansen, Brookfield Center, and Eric G. Quist, Roxhury, Conn., assignors to Mosler Research Products, Inc., Danbury, Conn., a corporation of Delaware Filed Sept. 21, 1966, Ser. No. 580,992 Int. Cl. (30% 13/00, 13/08; H03k 3/26 US. Cl. 340-276 14 Claims ABSTRACT OF THE DISCLOSURE A system for providing a central station with an indication of unauthorized activity at a remote protected area which includes a pulse generator at the remote area. The pulse generator transmits to the central station during nonalarm conditions pulses having pre-determined peak and base amplitudes falling within an acceptance band and having a specified pulse width to pulse spacing ratio. Should an intrusion occur the pulse width to spacing ratio of the generator output changes. A detector at the central station interconnecting the pulse generator and an alarm indicator monitors the pulse peak and base amplitudes to ascertain whether the pulses fall within the acceptance band and to thereby detect tampering, as well as monitors the average direct current level of the pulses to ascertain the existence of an intrusion.

This invention relates to alarm systems and more particularly to alarm systems of the type which sense unauthorized activity occurring in a protected area, such as forced entries and the like, and in response to such activity transmit instrusion signals to a remote central station for actuating an alarm, such as a buzzer or lamp, thereby alerting the central station personnel as to the existence of an alarm condition.

Alarm systems of the general type to which the present invention is directed typically include a sensing device located in the protected area, that is, in the area to be protected against unauthorized activity. The sensing device functions to detect the occurrence of unauthorized activities, such as unauthorized entries through a protected door, the unauthorized opening or breaking of a protected window, or the unauthorized movement or opening of a file, vault or safe. These detectors, while they may have a variety of constructions, commonly include a switch which is adapted to be actuated in response to the particular type of unauthorized activity such as the opening of a door. Others sense a change in an electrical condition, e.g. a change in capacitance due to an intruders approaching or touching a protected unit.

The systems of the general type to which this invention is directed also include, as an additional principal element, some kind of transmitting device under the control of the sensor for transmitting. an alarm signal to the central station in response to the sensing of unauthorized activity in the protected area. For convenience, the transmitter is typically located in the vicinity of the sensor. When an unauthorized event occurs, such as the opening of a protected door, the transmitter is actuated by the sensor, transmitting an alarm signal to the remotely located central station.

A further necessary component of this general type of alarm system is a receiver which is located at the central station. The receiver functions to detect the transmission of alarm signals for the purpose of actuating suitable alarm devices under its control, such as buzzers or lamps. The type and complexity of the receiver is subject to substantial variation depending upon the exact type of alarm 3,553,730 Patented Jan. 5, 1971 signal being transmitted in response to unauthorized activity.

In addition to means for causing an alarm signal in the event of instrusion, alarm systems of the present type also include means for continuously supervising the lines interconnecting the protected area and control station. These latter means are operative even during the normal day condition when some or all of the alarm devices are disconnected.

One of the principal problems with the prior art alarm systems is the provision of alarm and line supervisory signals whcih are effective to provide the central station with an indication that analarm condition exists. If these signals are of a relatively simple nature, e.g. a constant DC voltage, the system is vulnerable to compromise by substitution of a potential source, line termination or the like. On the other hand, if the security level of the system is raised by using more complex signals, this requires unduly complex and costly transmitting and receiving apparatus.

It has been, therefore, a principal object of this invention to provide an improved and simplified method of providing a central station with an indication that an alarm condition exists in the protected area, which greatly reduces the probability of system compromise and the consequent concealment of unauthorized activity.

This objective is achieved in the preferred embodiment of this invention by utilizing a fundamentally dilferent and novel concept in which the intrusion alarm signal takes the form of an abrupt and predetermined modification in the duty cycle, or width, of pulses in a train being transmited on a continuous basis from the remote area to the central station. More specifically, the intrusion alarm signal takes the form of a change in the ratio of the pulse width to the pulse spacing. For example, in one embodiment the pulses are of constant amplitude and have a normal, non-alarm width-to-space ratio of 3:1, which upon the occurrence of an intrusion in the protected area becomes modified to a ratio of 1:1. This change in the pulse width-to-space ratio or duty cycle comprising the intrusion alarm signal is elfective to produce an easily detected shift in the average or direct current level of the continuously transmitted pulses.

An advantage of using an intrusion signal of the above type is that the shift in the average signal level which it provides enables the structure and operation of the transmitter and receiver to be greatly simplified, resulting in a more economical and reliable system, as well as one which is not easily compromised.

It has been a further principal objective of this invention to provide a simplified method for determining the presence of an intrusion alarm signal among the pulses being continuously transmitted between the protected area and the central station. In the preferred embodiment of this invention, this objective is accomplished by first utilizing the unobvious step of converting the intrusion and rionintrusion pulses to low and high ranges saw-tooth wave forms, respectively, and then employing the difference in range of the wave forms so converted to selectively switch a transistor, thereby energizing a suitably connected buzzer or alarm.

An important advantage of the above technique for detecting the persence of intrusion alarm signals resides in the great simplification in receiver structure which is possible. For example, receivers for use in practicing this technique have been constructed utilizing, in addition to the transistor switch, little more than an integrating capacitor as the principal component. As those skilled in the art will appreciate, such simplification greatly advances the economies of alarm system fabrication, as well as their reliability and expected useful life.

A further objective of thisinvention has been to provide a simple method of detecting an attempted compromise of the system. This objective is achieved by a novel'and unobvious process which includes monitoring pulse base and peak amplitude levels and actuating a line alarm in response to base and peak amplitudes deviating from predetermined and arbitrary upper and lower limits defining an acceptance band. The line alarm, like the intrusion alarm, preferably includes a transistor switch adapted to energize a buzzer or similar device when actuated in response to the presence of pulses falling without the acceptance band.

One very important advantage of the above method is that it provides a high degree of protection against attempts to compromise the system, since any tampering with the lines or system circuitry in the protected area usually results in some detectable alteration in the amplitude of the pulse base or peak.

It has been a further objective of this invention to provide a simplified, yet sensitive, discriminator means useful in conjunction with the above method for monitoring the pulses in an effort to detect attempted system compromise. In accordance with the novel principles embodied in the present invention, the discriminator of the preferred embodiment is based on the concept of coupling the emitters of a pair of transistors, each responsive to the pulses, with a diode, and then biasing the transistors so that they switch at points corresponding to the desired upper and lower limits of the discriminator acceptance band. An advantage of a discriminator of this construction is that the width of the acceptance band is determined by the forward bias voltage drop of the diode and, consequently, is relatively small and invariant, allowing a high level of discrimination.

It has also been a principal objective of this invention to provide a simple, but yet reliable, pulse generator for transmitting the intrusion and nonintrusion pulse wave forms. This objective has been accomplished in the preferred embodiment of the present invention by providing a constant current source which alternately passes current through a pair of differently-valued resistive circuit paths connected across the generator output lines, the al teration in circuit paths being produced by an oscillator which functions to successively open and close a shunt switch located in one of the resistive paths. In this arrangement, the flow of current through the low resistance path having the shunt switch produces the pulse base, while the flow of current through the high resistance path produces the pulse peak. The width of the pulse base and peak is determined by the duration of current flow in each of the respective resistive paths, which in turn is controlled by the switching cycle of the shunt switch, the later being a function of the characteristics of the oscillator which controls its switching. Alteration of the pulse width and, hence, of the average direct current level or duty cycle of the intrusion alarm signal is effected by providing means for selectively varying the operational characteristics of the oscillator, this in turn being effective to vary the operation of the shunt swich located in one of the circuit paths.

In conjunction with the preceding objective, it has been a further objective of this invention to provide a simplified means for effecting the change in duty cycle of the pulses. This objective is achieved by providing a novel oscillator which includes a unijunction transistor and a capacitor. The capacitor, when it becomes fully charged, triggers the unijunction transistor, discharging therethrough for a predetermined time, thereby controlling the period for which the shunt switch is open-circuited and, hence, the width of the pulse. The pulse width is changed by merely altering the time required for fully discharging the capacitor.

An advantage of the above oscillator is that the period of capacitor charge and, hence, the period of unijunction transistor non-conduction and shunt switch closure,

is controlled by the resistance of the capacitor charge path, which in practice can be fixed, thereby maintaining the pulse spacing constant irrespective of the pulse width.

It has been a further objective of this invention, in conjunction with the preceding two objectives, to provide a pulse generator which has a relatively constant pulse base amplitude notwithstanding the existence of a shunt on the lines connecting the pulse generator and the receiver. This objective is achieved by connecting in series with the shunt switch a Zener diode which, upon actuation of the shunt switch, breaks down, providing a base amplitude equal to the Zener breakdown voltage. Since the Zener breakdown voltage is relatively insensitive to the current flowing through the diode, the amplitude of the pulse base which in reality is the breakdown voltage is held constant even in the presence of a shunt on the lines.

The various features and advantages of the invention will be more clearly apparent to those skilled in the art from the following description, taken in conjunction with the drawings.

In the drawings:

FIG. 1 is a simplified schematic circuit diagram of a preferred embodiment of a security alarm system constructed in accordance with the novel principles of this invention.

FIG. 2 is a detailed schematic circuit diagram of a pulse generator for generating intrusion and nonintrusion pulses which is suitable for use with the system of FIG. 1.

FIG. 3 is a detailed schematic circuit diagram of the alarm system of FIG. 1.

FIG. 4 is a plot of the intrusion and nonintrusion pulses produced by the generator of FIG. 2 showing the pulse peaks and bases and their relationship to the upper and lower limits of the acceptance band. This figure also shows the relative signal levels of the intrusion and nonintrusion sawtooth wave forms produced by the saw-tooth generator in response to the intrusion and nonintrusion pulses, respectively.

FIGS. 5 and 6 are plots of intrusion and nonintrusion wave forms which do not fall within the acceptance band of the discriminator.

FIG. 7 is a plot contrasting the nonintrusion pulses produced by the pulse generator during the daytime and nighttime.

FIG. 8 is a plot contrasting the intrusion alarm pulses produced by the pulse generator during the daytime and nighttime.

GENERAL DESCRIPTION The preferred embodiment of the system, as shown in FIG. 1, includes a pulse generator located in a protected area. The pulse generator 95 is selectively operable in either of two modes, namely, in either an intrusion alarm mode or in a normal, nonalarm mode. In practice, when an intrusion occurs as, for example, when an unauthorized entry is made through a protected door or window, a switch means is actuated in the pulse generator 95, switching its operation from the normal mode to the alarm mode. The two modes are distinguishable by their different average direct current output levels which result from their different pulse widths W and W the pulse spacings S being constant in either pulse mode (see FIG. 4). Preferably the width W of the pulse 53 produced in the alarm mode is a fraction, such as one-third, of the pulse width W of the pulse 51 produced in the normal, nonalarm mode. Hence, the switch in pulse generator mode occurring in response to unauthorized activity in the protected area, that is, in response to an intrusion alarm condition, is manifested by a decrease in the average direct current level of the generator output which is taken across lines 101 and 102.

The system also includes dual purpose detector circuitry. One purpose of the detector circuitry is to receive the generator output pulses on lines 101 and 102 and generate an intrusion alarm if the generator is in the intrusion alarm mode, thereby providing an indication at the central station of the existence of an intrusion in the protected area, such as an unauthorized entry through a protected door. The other function of the detector circuitry is to monitor the maximum pulse signal level, herein called the pulse peak, and minimum pulse signal level, herein called the pulse base, to detect departures relative to predetermined upper and lower limits and in response thereto produce what is termed herein as a line alarm. Such departures, or deviations, may result, for example, from attempts to compromise or otherwise tamper with the pulse generator in an effort to conceal unauthorized activity. Hence, fulfillment of this second function by the detector circuitry provides an indication at the central station of an attempt to conceal unauthorized activities in the protected area.

More specifically, as shown in FIG. 1, the detector circuitry includes a discriminator 107 having a high line voltage sensor 140 and a low line voltage sensor 141. The high line voltage sensor 140 is responsive to minimum pulse signal levels or pulse bases above, and maximum pulse signal levels or pulse peaks below, a predetermined upper limit, while the low line voltage sensor 141 is responsive to minimum pulse signal levels, or pulse bases, below a predetermined lower limit. These upper and lower limits, in combination, define an acceptance band. Those pulses having peak and base levels which do not actuate either of the sensors 140 or 141 are considered to fall within the band while pulses actuating one or more of the sensors are considered as falling without the band. Illustrative of pulses actuating the high voltage sensor 140 are pulses 64 and 67 (FIG. 5), which have bases above the upper band limit, and pulses 63 and 68 (FIG. 6), which have peaks below the upper band limit. Pulses actuating the low voltage sensor 141 are pulses 66 and 69 (FIG. 5), which have bases below the lower band limit.

Pulse signals within the acceptance band are gated on line 99 to a saw-tooth generator 110 where they are effective to produce either an intrusion saw-tooth signal 52 or a nonintrusion saw-tooth signal 50 (see FIG. 4) depending on whether intrusion pulses 53 or nonintrusion pulses 51 are being generated by the pulse generator 95 and gated to the saw-tooth generator 110. The intrusion saw-tooth signal 52 has a lower signal range than the nonintrusion saw-tooth signal 50 and is designed to actuate an intrusion alarm circuit 115 via lines 195 and 200. This intrusion alarm circuit 119 in turn generates outputs for actuating various alarms of alarm circuit 118. Specifically, an output is generated on line 216 for energizing an intrusion alarm lamp 119, and an output is generated on line 213 for energizing a buzzer 120. The nonintrusion saw-tooth signal 50 has a higher signal range and is designed to prevent actuation of the intrusion alarm circuit 115, and hence, actuation of the alarm circuit 118.

Pulse signals falling without the band as a result of the sensing by the sensors 140 and 141 of one or more of the line alarm pulse conditions to which they are responsive are effective to produce, respectively, high line voltage control signals on lines 132 and 134 and a low line voltage control signal on line 133. Specifically, the control signal on line 134, which is produced by the high voltage sensor 140 in response to pulses such as pulse 63, is input to the saw-tooth generator 110, disabling it, and thereby preventing either type of saw-tooth Wave form 50 or 52 from being generated on line 195. As a consequence of the absence of a saw-tooth signal on line 195, a line alarm circuit 130 is actuated, via line 131, producing outputs which actuate the alarm circuit 118. Specifically, an output is generated on line 230 which energizes a line alarm lamp 135 and an output is generated on line 231 which energizes the buzzer 120. The actuation of the line alarm circuit 130, in addition, actuates an inhibit circuit 96 via line 97 which, in turn, generates an inhibit signal on line 98 which is input to the intrusion alarm circuit to thereby prevent it from becoming actuated and energizing the intrusion alarm lamp 119.

The control signal on line 132, which is produced by the high voltage sensor 140 in response to pulses such as pulse 64, is also input to the line alarm circuit actuating the line alarm circuit. The actuation of the line alarm circuit operates as above to energize the alarm circuit 118 and inhibit the intrusion alarm circuit 115. Likewise, the control signal on line 133, produced by the low voltage sensor 141 in response to pulses such as pulse 66, is also input to the line alarm circuit 130 to thereby actuate it, in turn, energizing the alarm circuit 118 and inhibiting the intrusion alarm 115.

DETAILED DESCRIPTION The pulse generator 95, as shown more partiouarly in FIG. 2, includes a source of low voltage direct current potential 300. The source 300* is connected to a conversion circuit 301 for converting the low voltage D.C. potential to first and second stepped-up D.C. levels. The pulse generator 95 further includes a constant current regulator, generally indicated by the numeral 303. Two parallel circuit paths, generally indicated by the reference numerals 309 and 310, are fed by the constant current regulator 303 and provide high and low output signals across the output lines 101 and 102 in response to the alternate passing of current from the current source 303 through the respective paths 309 and 310. Successive high and low output signals produced across lines 101 and 102 correspond to the maximum signal or peak of a pulse and the minimum signals or base of a pulse, respectively. The duration of conduction through path 309 which produces a high or peak signal determines the width of the output pulse. Similarly, the duration of conduction through the path 310 which produces a low or base signal determines the pulse spacings.

An oscillator, generally indicated by the reference numeral 302, is connected to the circuit path 310 for alternately opening and closing this circuit path, producing, respectively, the high and low output signals on line 101 and 102 corresponding to the pulse peaks and bases. A switch 304 connected in the oscillator circuit 3-02 is provided for altering the oscillator output to thereby vary the time period that the circuit path 310 is closed and, hence, the width of the output pulse. A second switch 305 connected in the constant current regulator circuit 303 is included for varying the amplitude or peak of the output pulses. This switch makes it possible to use pulses having different amplitudes during the day and night. For example, the pulse generator may produce high peak pulses during the night and low peak pulses during the day (see pulses 60 and 61 of FIGS. 7 and 8, respectively).

'In operation, current from the low potential direct current source 300, following conversion by the circuit 301, is fed to the direct current source 303 and the oscillator 302, respectively. The oscillator 302 periodically interrupts the circuit path 310 allowing the current output from the constant current source 303 to alternately pass through the circuit path 309 and 310 producing the pulsing output across lines 101 and 102. Closing of the intrusion switch 304 alters the timing of the oscillator 302, in turn varying the period during which path 310 is open circuited, thereby altering the pulse width of the generator output.

More specifically, the low potential D'.C. source of supply 300 includes a positive terminal 315 and a negative terminal 316 which are connected to a ringing choke circuit generally indicated by the numeral 320. Specifically, the terminals 315 and 316 are connected, respec tively, to the midpoint 317 of a primary winding 318 of a step-up transformer 319 and to a junction 326. The ringing choke circuit 321 functions: in a well known manner to transform direct current into pulsating current.

The ringing choke circuit 320, in addition to the step-up transformer 319, includes a transistor Q having a collector 321 connected to one side of the primary winding 318, an emitter 322 connected to the negative terminal 316 of the DC). supply source 300 and to the other side of the transformer primary winding 318 via a diode 323 and a resistor 324. A capacitor 325, which also forms part of the ringing choke circuit 320, is connected across the emitter 322 and collector 321 of the transistor Q-15. The output of the ringing choke circuit 320 is taken across the secondary winding 330 of the transformer 319 at two dilferent points 335 and 347, providing two different pulsating current potentials in a manner well known in the art. Diode 332 having its cathode connected to terminal 331 and its anode connected to negative line 102, and diode 347 having its anode connected to terminal 347 and its cathode to line 348 rectify the pulsating ringing choke output, providing stepped-up DtC. conversion circuit outputs across lines 102 and 348, and lines 102 and 335. Capacitors 36 1 and 360 connected across output lines 348 and 102, and output lines 355 and 102, respectively, smooth the rectified pulsating current.

The oscillator 302 includes a unijunction transistor Q12 having a first base element 340 connected to the negative line 102. The unijunction transistor Q12 also includes a second base element 341 which is connected to the positive output terminal 335 of the transformer secondary winding 330 via a resistor 336. A control electrode 342 of the unijunction transistor Q-12 is connected to the cathode of the positive output terminal 335 via a resistor 343 and to a capacitor 344. The capacitor 344 at its other side is connected to the cathode of the positive output terminal 355 via a resistor 345. Also forming part of the oscillator 302 is a resistor 346 connected at one end to the junction of resistor 345 and capacitor 344, and at its other end to a second positive output terminal 348 of the transformer secondary winding 330 via the intrusion alarm switch 304 and the diode 307.

The constant current source 303 includes a transistor Q14 having an emitter 350 permanently connected to the positive output line 348 via a resistor 3 51 and selectively connected to the positive output line 348 via the night and day switch 305. The switch 305 is effective to selectively connect a resistor 368 is shunt with the re sistor .351 for increasing the amplitude of the output pulse peaks when closed. A base 352 of transistor Q-14 is connected to the negative output line 102 via a resistor 353 and to the positive output line 348 via a diode 355. A collector 356 of transistor Q-14 is connected directly to the positive output line 101. The parallel circuit paths 309 and 310 connected between the positive output line 101 and the negative output line 102 include resistor 357 and Zener diode 358 and the emitter-collector path of transistor Q13, respectively. The transistor Q-13 has its base 365 connected to the capacitor 344 and the resistors 345 and 346, its emitter 366 connected to the line 102, and its collector connected to the Zener diode 358.

In operation, direct current flows from the terminal 347 through diode 307, resistor 351 and the emittercollector path of transistor Q-14 constituting the constant current source, and thence alternately through the circuit paths 309 and 310 across which the output of the pulse generator is taken on lines 101 and 102. Current flowing through the load resistor 357 of the circuit path 309 produces a high signal level on output lines 101 and 102 of predetermined value. This signal level corresponds to the peak of the generator output pulse. The duration of this predetermined signal level, which determines the pulse width, is established by the period of nonconduction of the transistor Q-13 which in turn is established by the period of conduction of the unijunction transistor Q-12. In like manner, current flowing through the Zener diode 358 of circuit path 310 produces a low signal level on output lines corresponding to the base of the pulse. Because the Zener diode 358 has a constant voltage across it when in the breakdown mode, the base signal is substantially constant in level, thereby making the pulse generator relatively insensitive to shunts across the lines 101 and 102. If shunt sensitivity is desired, the Zener diode 358 may be replaced by a resistor. The duration of the base signal establishes the pulse spacing S of the generator output.

Until the capacitor 344 charges through resistor 343 to a point where it triggers unijunction transistor Q12, current flows through path 310, producing a pulse base, since the base circuit of transistor Q13 is biased to saturation causing current fiow through its emitter-collector path and, hence, through the path 310. The signal level across lines 101 and 102 at this time corresponds to the relatively low Zener breakdown voltage of diode 358 and, hence, constitutes the pulse spacing S. When the capacitor 344 charges to the triggering level of unijunction transistor Q-12, the unijunction transistor fires allowing capacitor 344 to discharge through the unijunction transistor Q-12, which is effective to lower the potential of the base of transistor Q13 driving transistor Q-13 into cut-olf. As transistor Q-13 is driven into cut-off, the constant current in the shunt path 310 established by the emitter-collector path of transistor Q14 is fed through resistor 357 of path 309, raising the output signal level on line 101. This raising of the output signal on line 101 corresponds to the peak of the output pulse and continues, establishing the pulse width, until the capacitor 344 has fully discharged through unijunction transistor Q-12. When this occurs, the unijunction transistor Q12 ceases conducting, causing the capacitor 344 to begin charging and transistor Q13 to be driven to saturation. The saturation condition of transistor Q-13 permits current to flow through path 310, lowering the output across lines 101 and 102 to the pulse base level established by Zener diode 358.

The change in pulse width of the pulses output on lines 101 and 102 is efiected by closing the intrusion switch 304. The closing of switch 304, which might occur in response to the unauthorized opening of a protected door, connects the resistor 346 into the oscillator circuit 302, allowing the capacitor 344 to discharge more quickly through the unijunction transistor Q12. Specifically, closing switch 304 connects resistor 346 into the discharge path of the capacitor 344, reducing the resistance of the path. This, in turn, allows the capacitor to discharge more quickly, reducing the period of conduction of the unijunction transistor Q-12. The reduced period of conduction of transistor Q-12 lessens the period of nonconduction of transistor Q-13. Hence, the pulse width, which corresponds to nonconduction of transistor Q13, is reduced.

The period for charging of the capacitor 344 and, hence, the period of conduction of the transistor Q-13, is the same regardless of whether intrusion switch 304 is closed or open. This results because the resistance of resistor 343 in the capacitor 344 charge path, which controls the charge time, is independent of the position of switch 304. Thus, the duration of conduction of transistors Q-13 and, hence, the period of Zener diode 358 breakdown, will be constant, in turn causing the pulse spacing S to be constant.

Summarizing, While closing the switch 304 is effective to decrease the discharge time for the capacitor 344, thereby shortening the pulse width, as, for example, from a width W to a Width W the closing of switch 304 is not effective to alter the charge time of the capacitor 344 to thereby shorten the pulse spacing S. Hence, the pulse spacing S remains constant in both the intrusion and nonintrusion modes of operation.

The increased amplitude night pulses and 61 are produced by closing switch 305. This places resistor 368 in shunt with the resistor 351, reducing the net resistance in the emitter-collector circuit of current regulator transistor Q-14. The reduced net resistance in this circuit permits increased current to fiow through the current regulator transistor Q-14. This raises the level of the signal across the output lines 101 and 102 when the current is flowing through path 309, raising the pulse peak. The increased current through path 310 during alternate cycles does not vary the signal level of the base portion of the pulse since the Zener breakdown voltage, which constitutes the base signal level, is not dependent on the current through the Zener diode 358.

The high voltage sensor 140 of the discriminator 1 07, as shown in FIG. 3, includes a transistor Q-l having a base 143, an emitter 144, and a collector 145. The base 143 is coupled to the discriminator input lines 101 and 102 via a resistor 146 connected to a tap 108 of a potentiometer 103 placed across the input lines. The emitter 144 is connected to a source of positive potential 147 via a potentiometer 148 and to the anode of a diode 173 having its cathode connected to an emitter 172 of a transistor Q3. The collector 145 is connected to a base 165 of a transistor Q2 via a coupling resistor 150 and to a collector 166 of a transistor Q-2 via a feedback capacitor 152. The collector 166 of transistor Q2 is further connected to a source of positive potential 151 via resistive elements 153 and 154. The emitter 104 of transistor Q-2 is connected to negative line 102.

The biasing of transistors Q-1 and Q-2 can be altered as desired by proper manipulation of potentiometers 103, 148 and 153. In practice, the bias is adjusted such that the transistors Q-l and Q-2 switch from saturation to cut-off when the instantaneous signal level of the input pulse rises above the upper band limit and switch from cut-oif to saturation when the instantaneous signal level of the input pulse falls below the upper band limit. Thus, the transistors Q1 and Q2 switch off and on in responce to the peak and base signal levels, respectively, of a pulse such as pulse 51, whereas the transistors Q-l and Q-2 are maintained in saturation and cut-off by pulses 63 and 64, respectively.

The high voltage sensor 140 also includes a Zener diode 155 having its cathode connected to the junction 156 of the feedback capacitor 152, the collector 166 of transistor Q-2, and the potentiometer 153, and its anode connected to a base 157 of a transistor Q- via a resistor 158 forming one-half of a voltage divider, the other half of which is a resistor 159 connected between the base 157 of a transistor Q-S and the negative line 102. A filtering capacitor 122 is connected between the negative line 102 and the anode of the Zener diode 155. The transistor Q-S further includes an emitter 160 which is also connected to the negative line 102, and a collector 161 upon which appears in response to a pulse such as pulse 64, a high line voltage control signal which is input to the line alarm circuit 130 via the lines 132 and 131.

In operation, if the pulse base input on line 101 to the discriminator 107 exceeds the upper band limit (see pulse 64 of FIG. 5), the transducer Q1 which is biased to switch from cut-off to saturation in response to the base of a pulse lying within the acceptance band will be driven to cut-off by an excessively large signal input to its base 143 and maintained in cut-off. This in turn causes transistor Q-2 to be driven into cut-01f due to the decrease in current in the emitter-collector path of transistor Q-l, which is reflected as a decrease in input to the base circuit of transistor Q-2. While the transistor Q-2, which like transistor Q-1 also switches in response to pulses in the acceptance band, is maintained in cut-01f, the decreased current flowing in its emitter-collector path raises the potential of collector 166 charging up the capacitor 190. When the capacitor 190 charges to the breakdown voltage of Zener diode 155, the Zener diode 155 breaks down passing current through resistors 158 and 159, thereby raising the signal level on the base 157 of normally cut-otr' transistor Q-S driving transistor Q-S into saturation. As transistor Q5 is driven into saturation, the impedance of its emitter-collector path approaches a negligible value completing a circuit between the negative line 102 and the input line 131 of the line alarm circuit 130. The completion of this circuit to the line alarm circuit from the negative line 102 effectively produces a high line voltage control signal on the high voltage sensor output line 132. This control signal on line 132, in a mnner to be described, actuates the line alarm circuit 130, which in turn energizes the line alarm lamp and the buzzer 120, and inhibits the intrusion alarm circuit 115 via the inhibit signal on line 97 generated by the actuated inhibit circuit 96.

The high voltage sensor also functions to actuate the line alarm circuit 130 to thereby energize lamp 135 and buzzer 120, should the pulse peaks not exceed the upper limit of the acceptance band. (see pulse 63 of FIG. 5). Specifically, if the pulse peak does not exceed the upper limit of the acceptance band, the transistor Q-l which is biased to switch to cut-off by the peak of a pulse exceeding the upper limit of the acceptance band will not switch to cut-off. Consequently, the transistor Q-l continues to operate in saturation. The continued conduction of the transistor Q-l results in continued conduction of transistor Q-Z which in turn causes the collector 166 of transistor Q-2 to be maintained at approximately the potential of negative line 102. This low potential on collector 166 constitutes a control signal on line 134 to the saw-tooth wave form generator 110, effectively disabling it. The disablement of saw-tooth generator 110 produces a low level input on line 195, actuating the .line alarm circuit 130, in turn energizing lamp 135 and buzzer 120 via lines 230 and 231, and inhibiting the intrusion alarm circuit 115 via a signal on line 97 from the actuated inhibit circuit '96.

The low voltage sensor 141 includes the transistor Q-3 having a base 170 connected to the discriminator input lines 101 and 102 via a resistor 171 connected to the tap point 108 of the potentiometer 103, the emitter 172 connected to the emitter 144 of transistor Q-l via the diode 173 and to the negative line 102 via resistor 174, and a collector 175. The collector 175 is connected to the inhibit. circuit 96 via a line 176, to the negative line 102 via a filtering capacitor 177, and to the high voltage side of the voltage divider comprising the resistors 178 and 179 separated by a tap point 180. The tap point 180 of the voltage divider is connected to the base 181 of a transistor Q-4 having an emitter 182 connected to the negative line 102 and a collector 183 connected to the junction of the resistor 164 and the diode 163 via the line 133.

The biasing of transistors Q-3 and Q-4 is adjusted such that the transistors both switch from cut-oil to saturation when the instantaneous signal level of the input pulse falls below the lower band limit, and switch from saturation to cut-off when the instantaneous signal level of the input ulse rises above the lower band limit. Thus, both of the transistors Q3 and Q4 switch on and on. in response to the base and peak, respectively, of a pulse such as pulse 66 (see FIG. 65).

In operation, if the minimum signal level or base of the pulse is below the lower limit of the acceptance band (see pulse 66 of FIG. 5), normally cut-off transistor Q3 is driven into saturation by the application to its base 170 of an excessively low signal via resistor 171. The increase in conduction of the transistor Q-3, because of the increase in current flow through its emitter-collector path, raises the potential of the divider tap point 180 causing an increased signal to be input to the base 181 of the transistor Q-4, driving normally cut-oil transistor Q-4 into saturation. The reduced impedance of the emitter-collector circuit of transistor Q-4 as a consequence of its being driven into saturation, completes a circuit between the negative line 102 and the line 133, eiIectively producing a control signal on line 133. This control signal, when coupled to the input 131 of the line alarm circuit 130 via the diode 163, actuates the line alarm circuit, in turn energizing the line alarm lamp 135 and buzzer 120, and inhibiting the intrusion alarm circuit 115 via a signal on line 97 from the actuated inhibitor circuit 96.

Pulses input to the discriminator 107 on lines 101 and 102 falling within the acceptable band established by the diode 173 in a manner to be described are gated through the high voltage detector circuit 140, effectively being reproduced at the junction 156 except for being clipped. That is, pulses input the discriminator 107 on lines 101 and 102 which fall within the acceptance band are reproduced at the junction 156 having the same pulse width and spacing and the same wave shape and timing. The reproduced pulses at junction 156 are clipped, however, due to the insensitivity of transistor Ql to the exact level of pulse peaks above the upper limit and the exact level of pulse bases between the upper and lower limits. Stated differently, any pulse input to transistor Ql having a peak above the upper limit and a base between the upper and lower limits causes the transistor Q-l and, in turn, the transistor Q-2, to switch, reproducing the pulse at junction 156. The reproduced pulse is a facsimile of the pulse applied to the base of transistor Q-1 to the extent of timing and pulse width-to-space ratio. This results regardless of the amount by which the applied pulse peak exceeds the upper limit or the exact position of the applied pulse base relative to the upper and lower band limits. This clipping action also permits the discriminator to respond equally well and in the same manner to both night and day pulses notwithstanding their difference in pulse amplitudes.

The diode 173 establishes the size of the acceptance band. Specifically, the forward biased voltage drop across the diode 173 determines the potential gap between the upper limit and the lower limit. As indicated, the switching of transistor Ql from saturation to cut-oil, which is necessary to avoid a line alarm, occurs when the instantaneous potential of the input pulse on lines 101 and 102 rises above the upper limit. Ignoring transistor junction voltage drops, this upper limit corresponds with the potential of the emitter 144 of transistor Ql. Similarly, the switching of transistor Q3 which is necessary to avoid a line alarm occurs when the instantaneous potential of the input pulse falls below the lower limit which, ignoring transistor junction voltage drops, corresponds to the potential of the emitter 172 of transistor Q3. Hence, it is the potential gap between the emitters 144 and 172 of transistors Q-1 and Q3, respectively, which defines the acceptance band. Since this potential gap is in reality the forward biased junction potential of the diode 173 which is connected between the emitters 144 and 172, the diode 173 is seen to in fact establish the width of the acceptance band.

The saw-tooth generator 110 includes an integrating capacitor 190 which is connected between the negative line 102 and the junction 156 via the parallel combination of a diode 191 and a potentiometer 192. The output of the saw-tooth generator is taken on line 195 connected to the junction of the capacitor 190, the diode 191 and the potentiometer 192. The capacitor 190 normally, that is, if there is no line alarm preventing the pulses input on lines 101 and 102 from being reproduced at junction 156, charges through the diode 191, the resistor 154, and the potentiometer 153, the latter controlling the charging rate, and discharges through the potentiometer 192, the setting of which controls the capacitor discharge rate.

More specifically, the presence of the reproduced pulses at the junction 156 cause the capacitor 190 to successively charge and discharge producing on line 195 a saw-tooth wave form, the maximum and minimum signal levels of which depend upon the pulse width-to-spacing ratio of the signal at junction 156 and, hence, on the signal present on lines 101 and 102. Specifically, if the pulse width-to-spacing ratio of the pulses on lines 101 and 102 and, hence, at junction 156 is high (see wave form 51 of FIG. 4), as is the case when no intrusion is present, the range between maximum and minimum saw-tooth pulse potentials is shifted upwardly (see saw-tooth wave form 50 in FIG. 4), as opposed to the range of the maximum and minimum levels (see sawtooth wave form 52 of FIG. '4) should the pulse widthto-spacing ratio be decreased (see pulse wave form 53 in FIG. 4), as is the case when an intrusion is present. Stated differently, the saw-tooth generator output wave form present on line 195 lies in an upper range if the pulse width-to-spacing ratio is high, as the case when no intrusion exists, and lies in a 'lower range if the pulse width-to-spacing ratio is reduced, as is the case when an intrusion has occurred.

The successive charging and discharging of the integratin capacitor 190, which is effective to produce either the upper range saw-tooth wave form 50 or the lower range saw-tooth wave form 52 corresponding to the absence and presence of an intrusion alarm, respectively, is interrupted by the presence of a high line voltage control signal on line 134. This temporary interruption of the charging of capacitor 190 is effective to generate a line alarm signal on line 195 which is input to the line alarm circuit via input line 131 to thereby actuate the line a'larrn circuit producing, in turn, illumination of the line alarm lamp 135 and the buzzer 120. Specifically, the low potential of the collector 166 of transistor Q2 constituting the control signal on line 134 causes the capacitor to discharge. The discharge of capacitor 190 lowers the signal level on line 195, in turn lowerin the input to the line alarm circuit 130 on line 131, actuating the line alarm circuit.

The intrusion circuit 115 which is responsive to the saw-tooth Waves present on line includes an input line 200 coupled to a base 2.01 of a transistor Q7 via resistors 203 and 204. Resistor 203, in combination with a resistor 199 connected between the negative line 102 and the resistor 203, forms a voltage divider having a tap point 202. The divider applies to the base circuit of the transistor Q7 via tap point 202 a fraction of the saw-tooth wave form voltage present on line 195 and input to the intrusion circuit via line 200. This enables the intrusion alarm circuit 115 to be actuated while leaving the line alarm circuit 130 unactuated, in a manner to be described, in response to the presence on line 195 of the lower range saw-tooth wave form 52 which exists under intrusion alarm conditions.

A capacitor 198 is connected in parallel with the resistor 199 and delays the switching of the transistor Q7 from saturation to cut-off as the signal level on line 195 drops, also for reasons to be described. The capacitor 198 also smooths the bias level input to the base circuit of transistor Q7 from line 195. The transistor Q7 also includes an emitter 205 connected via a diode 206 to the negative line 102, and a collector 207 connected to a source of positive potential 208 via a resistor 2.09. The intrusion alarm circuit 115 further includes a transistor Q-8 having a base 210 connected to the collector 207 of the transistor Q7, and an emitter 211 connected to the anode of diode 206 and to the emitter 205 of transistor Q7, and a collector 212 connected to the alarm circuit 118 via parallel circuit paths, the first of which includes a line 213, a diode 214, a line 215, and the second of which includes a line 216.

In operation, the biasing of transistors Q7 and Q8 is such that the saw-tooth wave form 50 present on line 195 and input to the intrusion alarm circuit 115 via line 200, which occurs in the absence of an intrusion alarm, biases transistor Q7 into saturation and transistor Q8 into cut-off. Whereas, if the wave form 52 is present on line '195, as occurs during the presence of an intrusion, the transistor Q7 is driven into cut-off raising the potential of collector 207, causing transistor Q8 to be driven into saturation. In the absence of an intrusion, transistor Q8 is cut-off and a high impedance is placed in circuit path between the negative 'line 102 and the parallel circuit paths defined by line 216, and lines 213, 215 and diode 214, which are input to the alarm circuit 118, thereby preventing the intrusion alarm indicating lamp 119 and the buzzer 120- from becoming energized. During an intrusion, transistor Q8 is driven into conduction and the large emitter-collector impedance of transistor Q8, which is normally in the negative line of the intrusion lamp alarm circuit 216 and the buzzer circuit 213, 214, 215, is removed. With this impedance removed, the negative line 102 is connected to the intrusion lamp .119 via diode 206, emitter-collector path of transistor Q-8, and line 216, and to the buzzer 120 via the diode 206, emitter-collector path of transistor Q8, line 213, diode 214, and line 215. With the negative line 102 effectively connected to the intrusion lamp 119 and the buzzer 12.0, the intrusion lamp 119' becomes illuminated and the buzzer 120 actuated, providing visual and audible indications of the intrusion alarm condition.

The line alarm circuit 130- includes a transistor Q9 having its base 220 connected to the input line 131 via a resistor 221, an emitter 222 connected to the negative line 102, and a collector 223 connected to a source of positive potential 224 via a resistor 225. A capacitor 219 is connected between the line 131 and the negative line 102. The capacitor 219 delays the switching of the transistor Q9 from cut-oif to saturation, following a line alarm condition, for reasons to be described later. The capacitor 219, like the capacitor 198, also smooths the input bias to the transistor Q9 from line 195. The line alarm circuit 130 also includes a transistor Q10 having a base 226 connected to the emitter 223 of transistor Q9 via a resistor 227, an emitter 228 connected to the negative line 102, and a collector 229 connected to a line alarm lamp 135 via a line 230, and to the buzzer 120 via line 231, diode 232, and line 215'.

In operation, the biasing of transistor Q9 and transistor Q10 is such that in the absence of a line alarm, that is, with the input pulses to the discriminator 107, such as pulses 51 or 53, located Within the acceptance band and either saw-tooth wave form 50 or 52 present on line 195, the capacitor 219 is charged maintaining transistor Q9 in saturation and transistor Q10 cut-off. With the transistor Q-10 cut off, a high impedance comprising the emitter-collector path of transistor Q-10 is placed between the negative line 102 and the line alarm lamp 135, and between the negative line 102 and the buzzer 120, preventing, respectively, the line alarm lamp 135 from becoming illuminated and the buzzer 120 from being actuated.

Should, however, the pulses input to the discriminator 107, such as pulses 63, 64, 66, 67, 68 or 69, fall without the acceptance band, producing either a high line voltage control signal on line 132 or 134 or a low line voltage control signal on line 133, as the case may be, the signal level on line 132, 134, or 133, as the case may be, decreases to a very low ,level producing a negative going input to the line alarm circuit 130 on line 131. The biasing of transistors Q9 and Q-10 is such that the discharge of capacitor 219 in response to this negative going pulse is effective to drive transistor Q9 into cut-off, in turn causing transistor Q10 to be driven into saturation. With transistor Q-10 driven into saturation, the high impedance presented by its emitter-collector path which occurs in the absence of a line alarm condition, is removed, effectively connecting the negative line 102 to the intrusion alarm lamp 135 via line 230 and to the buzzer 120 via line 231, diode 232, and line 215, causing the line alarm lamp 135 to become illuminated and the buzzer 120 to become actuated, respectively. Thus, both a visual and audible signal is produced at a control station indicating the existence of a line alarm condition.

By reason of the absence, in the line alarm circuit 130, of a voltage divider, such as that comprised of resistors 203 and 199 present in the input circuit of the intrusion alarm circuit 115, the line alarm circuit is not actuated by the change in saw-tooth generator output from saw- 14 tooth 50 to saw-tooth 52. Differently stated, while transistor Q7 does not remain conducting with Wave form 52 persent on line 195 due to the decrease in bias introducer bythe divider 2 03, 199, transistor Q9, which has no divider in its base circuit, does remain conducting. Thus, the divider 203, 199 renders the intrusion alarm circuit sensitive to an intrusion-induced switch from saw-00th wave form 50 to saw-tooth wave form 52, while the absence of such a divider in the line alarm circuit renders the line alarm circuit insensitive to such a change.

The inhibit circuit 96 includes a transistor Q-6 having a base 240 connected to the line 176 via a resistor 241 and a diode 242, an emitter 243 connected directly to the negative line 102 and a collector 244 connected to the junction of the base 210 of transistor Q-S, the collector 207 of transistor Q-7, and the low voltage side of resistor 209. A capacitor 197 is connected between the negative line 102 and the junction of the cathode of the diode 242 and the resistor 241. This capacitor delays the switching into cut-oil of transistor Q-6 in response to the return of the line alarm circuit 130 to its normal condition, allowing the potential on line 195 to increase before the inhibit circuit 96 is deactuated, thereby preventing a false actuation of the intrusion alarm circuit while the capacitor 190 is charging to the level corresponding to saw-tooth wave form 50. The inhibit circuit also includes a diode 245 which has its cathode connected to the junction of resistor 241 and the cathode of diode 242, and its anode connected via line 246 to the junction of collector 223 of transistor Q9, the low voltage side of resistor 225, and the resistor 227.

In operation, the inhibit circuit 96 prevents actuation of the intrusion circuit 115 should the line alarm circuit 130 be actuated. More specifically, actuation of the line alarm circuit 130 is accompanied by the driving of transistor Q9 into cut-off which raises the potential on the collector 223 of transistor Q9. This increased potential on the collector 223 of transistor Q9 is transmitted via the line 246 and the diode 245 to the base circuit of transistor Q6, driving normally cut-off transistor Q6 into conduction. The increased current flow through the emitter-collector path of transistor Q6, which accompanies the driving of transistor Q-6 into conduction, draws more current through the resistor 209 lowering the potential in the base circuit of transistor Q8, which in. turn drives transistor Q8 further into cut-01f, thereby preventing or inhibiting the actuation of the intrusion alarm circuit.

The switching of transistor Q-6 from cut-off to saturation, reducing the potential on the base 210 of transistor Q8 to thereby inhibit the intrusion alarm circuit 115, is further enhanced when a low line voltage is detected by the low line voltage sensor 141. More specifically, the detection of a low line voltage pulse input on lines 101 and 102 to the discriminator 107 by the sensor 141 is,

effective, as indicated previously, to drive transistor Q3' into saturation, raising the potential on the collector of transistor Q-3. The increased potential on the collector 175 of transistor Q-3 is applied to the base 240 of transistor Q6 via line 176, diode 242, and resistor 241, driving transistor Q6 into saturation. The switching of transistor Q6 to the conducting state is effective to inhibit the switching of transistor Q-8 and, hence, the actuation of the intrusion alarm circuit 115, in the same manner that the increased potential on the collector 223 of transistor Q9 in response to a line alarm is efiFective to inhibit the switching of transistor QS and the production of an intrusion alarm.

Prevention of false triggering of the intrusion alarm circuit following the cessation of a line alarm condition, which is provided by the capacitor 197 in the manner described previously, is further enhanced by the. capacitor 219. Specifically, this capacitor delays the returning of transistor Q9 to conduction following the termination of a line alarm condition which in turn delays the drop in potential of collector 223 and the subsequent deactuation of the inhibit circuit 96. These delays provide and opportunity for the saw-tooth wave form to reach the level of wave form 50 before the intrusion alarm becomes uninhibited, thereby preventing false intrusion alarm actuation while the potential on line 195 is rising to, but has not yet reached, the level corresponding to the nonintrusion level. In an analogous manner the capacitor 198 prevents false actuations of the intrusion alarm circuit 115 during the period that the potential on line 195 is dropping to the line alarm level and the inhibit circuit 96 is still unactuated. Specifically, capacitor 198 delays the switching of the transistor Q7 in response to a decreased input signal caused by a line alarm condition long enough for the transistor Q9 to switch and actuate the inhibit circuit 96. With the inhibit circuit 96 actuated, the intrusion alarm circuit 115 is unable to respond to the reduced signal present on its input circuit notwithstanding the expiration of the delay period established by capacitor 198.

The alarm circuit 118 includes the line alarm lamp 135 and the intrusion alarm lamp 119 which are connected in parallel between a source of intermittent positive potential 260 via line 261, diode 262, lines 263 and 264, and to the negative line 102 via line 230, collectoremitter path of transistor Q10 and via line 216 collector-emitter path of transistor Q8, and diode 206, respectively. The alarm circuit 118 also includes the buzzer 120 which is connected between the negative line 102 via line 265 and a positive source of DC potential 266 via lines 267 and 268, and the emitter-collector path of a transistor Qll, and a line 269. A resistor 285 connected between the lamps via line 264 and line 215 is eflectively connected in shunt across the lamps 119' and 135 permitting the buzzer 120 to become enegized notwithstanding the failure of one or more of the lamps 119 and 135. Transistor Q-11 has an emitter 270 connected directly to the source of positive. potential 266 via lines 267, 268, and a collector 271 connected directly to the buzzer 120 via line 269, and a base 272 connected to the line 215 via a filtering capacitor 273.

The alarm circuit 118 further includes a silencing circuit 280 which causes the lamps 119 and 135 to switch from a flashing state of illumination to a continuous state of illumination. Silencing circuit 280 includes a silicon controlled rectifier 281 having its anode connected to the source of positive potential 266 and to the emitter 270 of transistor Q-11, and its cathode connected to the indicator lamps 119 and 135 via line 264 and to the resistor 285. The gate element 282 of the silicon controlled rectifier 281 is connected to a source of positive gating potential 283 via a switch 284, a diode 279, and a resistor 286.

In operation, the intrusion alarm lamp 119 becomes illuminated when the intrusion alarm circuit 115 is actuated in response to the increased pulse repetition rate which results when an intrusion exists in the protected area. More specifically, the intrusion lamp 119 becomes illuminated when the negative line 102 is coupled to the intrusion lamp line 216 via the emitter-collector path of transistor Q8, the diode 206, the emitter-collector path of transistor Q8 having low impedance only when tran sistor Q8 has been driven into saturation in response to the actuation of the intrusion alarm circuit 115. When the negative line 102 has been coupled to the intrusion lamp line 216, an enegization circuit is completed to the lamp, energizing the intrusion lamp 119 in a flashing mode.

The line alarm lamp 135, which is also directly connected to the source in intermittent DC potential 260 via lines 264, 263, diode 262, and line 261, becomes illuminated when its negative line 230 is connected to the negative line 102. This completion of the energization circuit for the line alarm lamp 135 is effected by connecting line 230 to the negative line 102 via the low impedance emittercollector path of transistor Q10 which exists when transistor Q-110 has switched to the high conduction state in response to the line alarm circuit caused by a low line vo tage or a highline voltage signal input to the discriminator 107. The source of intermittent positive potential 260 is continuously coupled to the lamp line 264 via the line 261, diode 262, line 263, and consequently a lamp energization circuit is completed when the negative line 102 has been connected to the appropriate lamp 119 or 135.

The buzzer 120 is also actuated by the switching of transistors Q8 and Q-10 in response to the actuation of the intrusion alarm circuit and/or the line alarm circuit 130, respectively. Specifically, the switching of transistor Q8 to its high conduction state couples the base circuit including line 215 of transistor Q-11 to the nagative line 102 via the diode 214, the low impedance emitter-collector path of transistor Q8, and the diode 206, thereby switching transistor Q-11 to a conducting state. The switching of transistor Q11 to a conducting state reduces the emitter-collector impedance of transistor Q11, eifectively connecting the continuous source of DC potential 266 to the buzzer via line 268, emitter-collector path of transistor Q-11, and the line 269. The coupling of the source of continuous positive potential 260 to the buzzer completes the energization path for the buzzer, thereby actuating the buzzer, the buzzer being directly and continuously connected to the negative line 102 via the line 265. Due to the capacitive coupling of the base 272 of the transistor Q-ll to the completed negative line circuit, transistor Q11 ceases to conduct when the capacitor 273 charges, thereby providing only a pulse of current to the buzzer 120. The buzzer 120 remains operative, however, due to the inclusion therein of latching means. Suitable latching means may be of the tpye disclosed in application Ser. No. 576,295, in the name of Donald E. Hansen, for a Security Alarm System, filed on Aug. 31, 1966.

The buzzer is similarly actuated when the transistor Q10 is driven into conduction in response to the line alarm circuit. Specifically, the low impedance emittercollector path of transistor Q10, which results when this transistor is driven into conduction in response to the actuation of the line alarm circuit 130, effectively connects the negative line 102 to the base circuit of the transistor Q-11 via line 231, diode 232, and line 215. The connection of the negative line 102 to the base circuit of transistor Q-10 of the line alarm circuit drives the transistor Q11 into conduction which, in turn couples the continuous source of DC potential 266 to the buzzer 120 via the reduced impedance emitter-collector path of transistor Q-11. Thus, the buzzer 120 becomes actuated in response to the actuation of either the intrusion alarm circuit 115 or the line alarm circuit 130, which effectively couples the negative line 102 to the base circuit of the transistor Q11 which, upon the switching, connects the buzzer 120 to a source of positive potential 266.

The intermittent energization or flashing state of the lamps 119 and may be switched to a continuous state of energization by closing the silence switch 284. ,Specifically, the switch 284 when closed couples a source of positive potential 283 to a gate electrode 282 of a silicon controlled rectifier 281 via a diode 279 and a resistor 286, which is effective to trigger the silicon controlled rectifier, causing it to conduct. The conduction of the silicon controlled rectifier 281 in response to closing the silence switch 284 couples the continuous source of DC potential 266 to the lamps 119 and 135 via line 267, silicon controlled rectifier 281, and line 264. This causes the lamps to receive continuous energization, thereby remaining illuminated continuously as opposed to intermittently, which is the case prior to the energization of the silicon controlled rectifier when the line 264 is coupled to the intermittent source of DC otential 260. An R-C filter network 280 connected in the gate circuit of rectifier 281 is included to prevent false triggering of the rectifier in response to transients.

OPERATION The circuit of FIG. 1 has four principal modes of operation, namely, a normal mode, an intrusion alarm mode, and high and low line alarm modes. The normal mode of operation results in the absence of either an intrusion alarm or a line alarm. The intrusion alarm mode occurs when the pulse width-to-spacing ratio decreases in response to an intrusion alarm. The high line voltage and low line voltage alarm modes exist when the pulses input to the discriminator 107 do not fall within the acceptance band for one or more reasons.

In the normal mode of operation, the pulses from the generator 95 input to the discriminator 107 lie within the acceptnace band. That is, they have a maximum signal level or peak above the upper limit of the acceptance band, and a lower signal level or base between the upper and lower limits of the acceptance band (see pulse 51). In addition, the pulse width-to spacing ratio is approximately 3: 1. With the pulses 51 lying Within the acceptance band, neither the high voltage sensor 140 nor low voltage sensor 141 is actuated, and the pulses are gated through the discriminator circuit 107 to the saw-tooth generator 100, producing an upper range saw-tooth wave form on line 195. This wave form 50 on line 195 biases both transistor Q7 of the intrusion alarm circuit 115 and transistor Q9 of the line alarm circuit 130 into saturation, which in turn maintains transistor Q8 of the intrusion alarm circuit 115 and transistor Q-10 of the line alarm circuit 130 in cut-off, thereby decoupling the negative line 102 from the intrusion alanm lamp 119, the line alarm lamp 135, and the buzzer 120 of the alarm circuit 118, preventing their energization. The same circuit operation results if normal night pulses are input to the discriminator 107 inasmuch as the gating circuitry, including transistors Q-1 and Q2, operate in a clipping mode, being insensitive to the exact amplitude of pulse peaks which exceed the upper limit.

In summary, in the normal mode of operation, the pulses 51 from the generator input to the discriminator 107 are gated through the discriminator to the saw-tooth generator where a high range saw-tooth wave form 50 is generated. This wave form is effective to maintain the intrusion alarm circuit and the line alarm circuit in a deactuated state, which in turn prevents the negative line 102 fnom being coupled to the alarm circuit 118, thereby preventing actuation of any of the alarm devices of the alarm circuit. Hence, in the normal mode of operation, no alarm device is actuated at the central station.

In the intrusion mode of operation, the pulses input to the discriminator 107, while lying Within the acceptance band of the discriminator 107, do undergo a change in the pulse width-to-spacing ratio. Specifically, when an intrusion occurs in the protected area, the output pulses of generator 95 undergo a change from a width-to-spacing ratio of 3 :1 to a width-to-spacing ratio of 1:1, that is, the wave form changes from that of pulse 51 to that of pulse 53. The reduced ratio pulses input to the discriminator 107 are gated through the discriminator 107 since they lie within the acceptance band and are input to the sawtooth generator 110. In view of the decreased pulse width of the input to the generator 110, the capacitor 190 discharges more frequently, lowering the range of the sawtooth wave form present on line 195 to that of wave form 52. The reduced level of the saw-tooth wave form 52 present on line 195 is insufiicient to maintain transistor Q7 of the intrusion alarm circuit 115 in its normally conducting state and, hence, transistor Q7 is switched to cutoff which in turn switches transistor Q-S into conduction.

Transistor Q9 of the line alarm circuit 130 is not also driven into cut-off in response to the reduced range of saw-tooth wave form 52 present on line 195 because of the absence of a voltage divider in its base circuit of the type which is present in the base circuit of transistor Q7. The voltage divider including resistors 203 and 199 of the intrusion alarm circuit 115 applies only a fraction of the signal present in line 195 to the base circuit of the transistor Q7, whereas the full signal present on line 195 is applied to the base circuit of transistor Q9 of the line alarm circuit 130. That is, for any given signal level on line 195, the input to the base of transistor Q7 is below the input to the base of transistor Q9. By pr0p erly biasing transistors Q7 and Q9, it is possible to make the divided saw-tooth wave form 52 applied to the base circuit of transistor Q-7 low enough to switch transistor Q7, while at the same time make the undivided sawtooth wave form 52 applied to the base circuit of transistor Q9 high enough to prevent transistor Q9 from switching. Consequently, the intrusion alarm circuit 115, in response to the reduced range saw-tooth signal 52 present on line 195 becomes actuated, Whereas the line alarm circuit 130 does not.

With the transistor Q7 of the intrusion alarm circuit driven into cut-off and transistor Q8 driven into conduction, a low impedance path exists between the negative line 102 and both the intrusion alarm lamps 119 and buzzer 120. Specifically, with the transistor QS in the high conduction state in response to the reduced range wave form 52 present on line 195, the negative line 102 is coupled through diode 206 and the low impedance emitter-collector path of transistor QS to the intrusion alarm lamp 119 via line 216 and to the buzzer 120 via line 213, diode 214, and line 215. This coupling of the negative line 102 to both the intrusion alanm lamp 119 and the buzzer 120 completes the energization circuit to the intrusion alarm lamp 119 and buzzer 120, causing them to become actuated. Specifically, the intrusion alarm lamp becomes visibly flashing and the buzzer begins pro viding an audible signal indicating the existence of an intrusion alarm.

The above circuit operation also exists if night intrusion pulses 61 are input to the discriminator 107, due to the clipping action of transistors Q-l and Q2 discussed previously.

In summary, the occurrence of an intrusion alarm in the protected area causes the generator 95 to produce pulses 53 or 51 having a decreased width-to-spacing ratio which, assuming that they fall within the acceptance band of the discriminator 107, are gated to thesaw-tooth generator, producing a reduced range of saw-tooth signals 52. The reduced range of saw-tooth signals 52, due to voltage divider action, is effective to actuate only the intrusion alarm circuit 115. This, in turn, completes an energization path to the intrusion alarm lamp 119 and buzzer 120, producing at the central station both an audible and a flashing indication of the occurrence of an intrusion alarm in the protected area.

In the high voltage line alarm mode of operation, the high voltage sensor operates in one of two sub-modes depending on whether the sensor 140 is detecting a pulse having a peak below the upper limit of the acceptance band (see pulse 63) or a pulse having a base exceeding the upper limit (see pulse 64). In either sub-mode, the width-to-space ratio of the pulses is 3:1 characteristic of the normal mode of operation. if a pulse such as pulse 63 is input to the discriminator 107, a first sub-mode results, producing a control signal on line 134 which disables the saw-tooth generator 110 and actuates the line alarm 130. In a second sub-mode, a pulse such as pulse 64 produces a control signal on line 132 which also actuates the line alarm 130.

More specifically, the high voltage sensor 14w operates in the first sub-mode, producing a control signal on line 134, in response to the presence of pulse 63 which fails to switch transistors Q1 and Q2, allowing them to remain conducting. With transistor Q2 conducting, its collector 1 66 is maintained at a low potential, producing a low level signal on line 134 constituting the control signal. This low level control signal on line 134 is input to the saw-tooth generator 110, effectively disabling it by allowing the integrating capacitor to discharge. The discharged capacitor 190 produces a low potential signal on line 195, supplanting the saw-tooth wave forms 50 or 52 which are present in the absence of a line alarm condition. This low potential on line 195 is reflected at the input to the line alarm circuit 130 on line 131 via resistor 164 and diode 163, and causes the capacitor 219 in the base circuit of transistor Q9 to discharge through the base-emitter path of transistor Q9, switching normally saturated transistor Q9 into cut-oft. The switching of transistor Q9, in turn, switches transistor Q-10 from cut-off into saturation. With transistor Q-10 saturated, its emitter-collector path is now at a low impedance, effectively coupling the negative line 102' to the line alarm lamp 135 via line 230 and to the buzzer circuit via line 231, diode 232, and line 215, energizing the lamp 135 in a flashing mode and latching the buzzer 120, respectively.

In addition to actuating the lamp .135 and buzzer 120, the actuation of the line alarm circuit also energizes the inhibit circuit 96. Specifically, the rising collector potential of transistor Q9 caused by its switching to cut-off is applied via line 246 and diode 245 to the base circuit of transistor Q-6, switching transistor Q6 to saturation from cut-ofl. The conduction of transistor Q6 draws more current through resistor 209, lowering the base potential of transistor Q-S, thereby preventing transistor Q8 from switching to saturation and actuating the intrusion lamp 119.

The high voltage sensor 140 operates in the second sub mode producing a control signal on line 132 and actuating the line alarm circuit 130 in response to the presence of a pulse such as pulse 64 input to the discriminator 107. More specifically, with pulse 64 input to the discriminator 107, the transistors Ql and Q2 fail to switch, remaining cut-01f thereby maintaining a high potential at junction 156. This high potential charges up the capacitor 190. When the potential at point 156 exceeds the breakdown voltage of Zener diode 155, current passes through the Zener diode, raising the potential at the base 157 of transistor Q-5, which switches normally cut-off transistor Q- into saturation. The switching of transistor Q-S produces a high line voltage control signal on line 132. This control signal is approximately at the potential of the negative line 102 due to the low impedance of the emittercollector path of conducting transistor Q5, and when applied to the base circuit of transistor Q9 of the line alarm circuit 130 is eflective to cause the capacitor 219 to discharge through the transistor Q-'5. The discharge of capacitor 219 drives normally saturated transistor Q9 into cut-off. This, in turn, drives normally cut-oil transistor Q10 into saturation.

With transistor Q10 conducting, a low impedance is inserted between the negative line 102 and the line alarm lamp 135 and buzzer 120. Specifically, the low impedance of the emitter-collector path of transistor Q10 couples the negative line 102 to the line alarm lamp 135 via line 2'30 and to the buzzer 120 via line 215, diode 232, and line 231, completing an energization path to the line alarm lamp 135 and the buzzer 120, respectively. The completion of these energization paths illuminates the line alarm lamp 135 in a flashing mode and causes an energization pulse to be applied to the buzzer 120, causing it to latch and provide a continuous audible signal.

An intrusion alarm is not also produced due to the disablement of the intrusion alarm circuit 115 in response to actuation of the line alarm circuit 130, for reasons described previously.

In summary, when the pulse peak falls below, or the pulse base exceeds the upper band limit, the high line voltage sensor 140 functions in first and second submodes, respectively, producing control signals on lines 134 and 1132, respectively. These signals, in turn, produce low potential inputs on line 131 of the line alarm circuit 130, svw'tching the transistors therein, thereby completing energization circuit paths to both the line alarm lamp 135 and the buzzer 120*, and inhibiting the intrusion alarm circuit 115.

In the low line voltage alarm mode of operation, the generator 95 produces pulses having the no m l 3:1 p lse width-to-spacing ratio. However, the minimum signal level or base of the pulses produced falls below the lower limit of the acceptance band of discriminator 107 (see pulse 66). With such pulses being input to the discriminator 107, the low line voltage sensor 141 becomes actuated.

Specifically, the low voltage signals drive normally cutolf transistor Q3 into conduction which in turn applies an input signal to the base circuit of transistor Q 4 driving normally cut-off transistor Q4 into conduction. The switching of transistor Q4 of the low line voltage sensor 141 into conduction produces a low line voltage control signal on line 133. This low potential is reflected at the input line 131 of the line alarm circuit 130, permitting the capacitor 221 to discharge through the transistor Q9 and drive normally saturated transistor Q9 to cut-off which, in turn, drives normally cut-off transistor Q10 into conduction. With transistor Q10 conducting, energization paths are completed to the line alarm lamp 135 and to the buzzer in the same manner as described previously with respect to the actuation of the line alarm circuit in response to high line voltage alarm conditions.

The above circuit operation of the low voltage sensor 141 results if intrusion pulses 69 having their bases below the lower band limit are input to the discriminator 107. However, no intrusion alarm results due to the intrusion alarm being inhibited by the actuated condition of the line alarm.

In summary, when the minimum signal level or base of the pulses input to the discriminator 107 fall below the lower limit of the acceptance band, the low line voltage sensor 141 becomes actuated, producing a low line voltage control signal on line 133. This in turn is effective to produce a low potential line alarm signal on line 131 of the line alarm circuit 130, switching the transistors therein which are effective to complete energization circuit paths to both the line alarm lamp 135 and the buzzer 120, and inhibit the intrusion alarm circuit 115.

We claim:

1. A system for providing a central station with an indication of unauthorized activity occurring at a remote protected area, said central station and remote protected area being linked by signal transmission means, said system comprising:

a variable duty cycle signal generator means located in said remote area operable to produce varying duty cycle signals, said generator being operable in a normal mode in the absence of said unauthorized activity for producing a time varying output signal having a first average level, and operable in an alarm mode in the presence of said unauthorized activity for producing an output signal having a second average level different from said first level, said signal generator means being located entirely at said remote station and operable in said alarm mode independently of tampering-induced changes in said transmission means;

an unauthorized activity sensor located entirely at said remote protected area for placing said signal generator in said alrm mode in response to the detection of said unauthorized activity;

an alarm indicator located at said central station for providing an indication of unauthorized activity when actuated; and

a detector interconnecting said signal generator and said alarm indicator for actuating said alarm indicator in response to the receipt of said output signal having said second average level.

2. A system for providing a central station with an indication of an alarm condition at a remote area, said system comprising:

variable duty cycle signal means at said remote area for generating varying duty cycle signals, said generator being operable to produce a first time varying output signal in the form of a variable pulse width rectangular waveform having a first average DC level and for generating a second time varying output signal in the form of a variable pulse width rectangular waveform having a second average DC level diiferent from said first level, said signal generator means being located entirely at said remote station and operable in said alarm mode independently of tamperinginduced changes in said transmission means;

an alarm condition sensor located entirely at said remote area for operating said signal means to produce said first and second average level output signals in the absence and presence of said alarm condition, respectively;

an alarm indicator located at said central station for providing an indication of an alarm condition when actuated; and

a detector interconnecting said signal means and said alarm indicator for actuating said alarm indicator when said second level signal is output from said signal means.

3. A system for providing a central station with an indication of unauthorized activity occurring at a remote protected area, said central station and remote protected area being linked by signal transmission means, said system comprising:

variable duty cycle signal means at said remote area for generating varying duty cycle signals, said generator being operable to produce a time varying output signal having a first average level in the absence of said unauthorized activity, and an output signal having a second average level different from said first level in the presence of said unauthorized activity, said signal generator means being located entirely at said remote station and operable in said alarm mode independently of tampering-induced changes in said transmission means;

an unauthorized activity sensor located entirely at said remote protected area controlling said signal means for producing said first and second average level signals in the absence and presence of said unauthorized activity, respectively; and

a detector at said central station responsive to said signal means for providing an alarm indicating signal when said second level signal is output from said signal means.

4. The system of claim 3 wherein said time varying output signal is a pulse signal.

5. A system for providing a central station with an indication of unauthorized activity occurring at a remote protected area, said central station and remote protected area being linked by signal transmission means, said system comprising:

a variable duty cycle signal generator means operable to produce varying duty cycle signals, said generator being operable in an alarm mode in response to said unauthorized activity for producing time varying output signals having a first average direct current level, and operable in a normal mode in the absence of said unauthorized activity for producing time varying output signals having a second average direct current level difierent from said first level, said signal generator means being located entirely at said remote station and operable in said alarm mode independently of tampering-induced changes in said transmission means;

an unauthorized activity sensor located entirely at said remote protected area for operating said signal generator in said alarm mode in response to the detection of unauthorized activity;

an alarm indicator located at said central station for providing an indication of unauthorized activity when actuated; and

a detector interconnecting said signal generator and said alarm indicator for actuating said alarm indicator in response to the receipt of said time varying output signals having said first average direct current level.

6. The system of claim 5 wherein said signal generator means includes circuit means for causing said time varying output signals produced by said signal generator When operating in said alarm and normal modes to comprise pulses having adjacent base and peak signal portions.

7. The system of claim 6 wherein said signal generator means includes circuit means for causing the width ratio of said peak and base portions of said alarm mode pulses to be different from the width ratio of said peak and base portions of said normal mode pulses.

8. The system of claim 6 wherein said signal generator means includes circuit means for causing the pulses produced during said alarm mode to have substantially the same peak and base amplitudes as the peak and base amplitudes of the pulses produced during said normal mode.

9. The system of claim 8 wherein said detector includes a capacitor for producing difierently ranged alarm and normal sawtooth wave forms in response to said alarm and normal mode pulses, respectively, and wherein said detector further includes a switch responsive to said alarm and normal mode saw-tooth wave forms and capable of distinguishing therebetween for selectively actuating said alarm indicator in response to said alarm mode saw-tooth wave form.

10. The system of claim 9 wherein said detector further includes a discriminator for actuating said alarm indicator in response to the deviation of pulse amplitudes from a predetermined level.

11. The system of claim 10 wherein said signal generator includes:

a regulated constant current source;

first and second circuit means connected in parallel to said current source at a common terminal, and

cyclic means for periodically altering the impedance value of at least one of said impedances for producing a time varying signal at said common terminal.

12. The system of claim 8 wherein said detector includes a discriminator for actuating said alarm indicator in response to deviations of said pulse amplitudes from a predetermined level.

13. The system of claim 12 wherein said discriminator actuates said alarm indicator in response to pulse base amplitudes falling without an acceptance band defined by upper and lower band limits and in response to pulse peak amplitudes below said upper limit.

14. The system of claim 12 wherein said alarm indicator includes a line alarm device responsive to said discriminator for indicating the presence of pulse amplitudes deviating from said predetermined level, and further includes an intrusion alarm device responsive to said detector for indicating receipt of said alarm mode pulses by said detector.

References Cited UNITED STATES PATENTS 3,135,951 6/1964 Byrne 340-'276 3,312,107 4/1967 Burns et a1. 340-206 3,375,493 3/1968 Gottlieb 331-111X ALVIN H. WARING, Primary Examiner J. M. BOBBITT, Assistant Examiner US. Cl. X.R. 33llll; 340274 

